Diagnostic Criteria for Rhabdomyolysis
The primary diagnostic criterion for rhabdomyolysis is an elevation of serum creatine kinase (CK) activity of at least 10 times the upper limit of normal followed by a rapid decrease to near-normal values. 1
Core Diagnostic Features
- Serum creatine kinase (CK) elevation ≥10 times the upper limit of normal is the biochemical "gold standard" for diagnosis 2, 1, 3
- Myoglobinuria, manifesting as red-to-brown urine, is a characteristic finding and considered the "gold standard" for prognostication 2, 4
- The classic clinical triad includes muscle pain, weakness, and dark urine, though this is present in less than 10% of cases 1, 5
Laboratory Evaluation
- Complete electrolyte panel to assess for hyperkalemia, hypocalcemia, and hyperphosphatemia, which are common complications 6
- Renal function tests (BUN and creatinine) to monitor for acute kidney injury, which occurs in 13-50% of cases 6, 2
- Liver function tests to assess for hepatic involvement and distinguish between hepatocellular and other patterns of injury 6
- Coagulation studies to evaluate for disseminated intravascular coagulation, a potential complication of severe rhabdomyolysis 6
- Arterial blood gas analysis to assess for metabolic acidosis 6
- ECG and cardiac troponin to rule out cardiac involvement, especially in severe cases 6
Monitoring Protocol
- For severe rhabdomyolysis (CK >5,000 IU/L), monitor CK, electrolytes, and renal function every 6-12 hours initially 6
- Once CK levels begin to decrease, monitoring can be reduced to daily 6
- Serial CK measurements are important to track the course of rhabdomyolysis 6
- Risk of acute kidney injury significantly increases when CK levels exceed 16,000 IU/L 1
Special Considerations
- In traumatic cases, assess for compartment syndrome (pain, tension, paresthesia, and paresis) 6
- In cases of exertional rhabdomyolysis, recurrent rhabdomyolysis, or persistently raised serum CK with no identified cause after neurological work-up, consider referral for investigation of malignant hyperthermia susceptibility 7
- For patients with crush syndrome, CK levels above 75,000 IU/L are associated with high risk (>80%) of acute kidney injury 6
Etiology-Specific Testing
- Viral studies should be performed if viral myositis is suspected 6
- Autoimmune markers (ANA, ASMA, ANCA) should be tested if autoimmune myositis is suspected 6
- EMG and MRI may be useful in cases of inflammatory myositis 6
- In cases of recurrent rhabdomyolysis, consider testing for hereditary muscle enzyme defects 4
Risk Factors to Consider
- Medications, particularly statins (especially when combined with macrolides, cyclosporine, antifungal drugs, or niacin) 8
- Excessive muscle activity, especially novel overexertion or unaccustomed exercise 8
- Prolonged muscle compression during surgery or immobilization 8
- High-temperature environments (above 80°F/27°C) during exercise 8
- Dehydration can increase risk and worsen symptoms 8
Common Pitfalls and Caveats
- The classic triad of symptoms (muscle pain, weakness, and dark urine) is present in less than 10% of patients, so absence of these features does not rule out rhabdomyolysis 1
- Mortality rate is approximately 10% and significantly higher in patients who develop acute renal failure 1
- Delay in diagnosis and treatment increases risk of complications, particularly acute kidney injury 2, 3
- Rhabdomyolysis can occur without obvious trauma or exertion, so maintain a high index of suspicion in at-risk patients 4
- In patients with recurrent episodes, consider underlying genetic disorders 1